1. Field of the Invention
This invention relates to the bonding of ceramic, semiconductor and metallic materials to each other, and more particularly to securing an array of environmental sensors and their associated lead wires to a common substrate.
2. Description of the Related Art
In applications such as the semiconductor and liquid crystal display (LCD) manufacturing industries, it is necessary to sense the absolute value and uniformity of environmental factors, such as temperature, radiation, pressure and gas composition and/or concentration, over a given surface area. To this end a plurality of sensors are disposed on a substrate which has the desired area and is placed in the location where sensing is needed. Each sensor, which is typically pre-calibrated to correlate with the values of the sensed quantity, is connected by lead wires to a remotely located response facility which responds to the sensor output, such as by providing a readout of the sensed environmental characteristic and/or controlling other elements of the manufacturing process.
In semiconductor and LCD manufacturing, the absolute temperature and temperature uniformity in processing equipment are important parameters in establishing manufacturing yields. Such processing equipment includes furnaces (such as rapid thermal process, tube, belt and tray furnaces), vacuum sputtering and evaporation equipment, chemical vapor deposition reactors, plasma, reactive ion and wet chemical etchers, the application and removal of photo resist and glass for spinners, chill plates, hot plates and strippers, and wafer and plate chucks used for device and integrated circuit (IC) testing.
The thermal sensors are typically type K, R or S thermocouples (TCs) or thin film platinum resistance temperature detectors (TFRTDs). The TC junctions or TFRTDs are typically mounted within respective cavities in the substrate, and secured within the cavities by a bonding material. The bonding material for TCs is commonly a mixture of SiO2 and Al2O3, typically 60 wt. % SiO2 and 40 wt. % Al2O3. TFRTD sensors typically require two cavities each, one for the sensor itself and the other to strengthen lead wire bonding, with the bonding material for both cavities typically an epoxy.
The electrical lead wires for the sensors are typically sheathed in quartz microtubing segments having a maximum temperature rating of 1100° C., braided silica sleeving having a maximum temperature rating of 1100° C., or Teflon® sleeving having a maximum temperature rating of 250° C. The lead wires extend from their respective sensors and are commonly bundled together, with the bundling commencing either on or off the substrate.
Cavities are necessary for accurate temperature calibration of TCs because the TC junctions, plus at least about 2.5 cm of lead wire extending from the junction, must be inside the temperature measurement volume for accurate measurements. Cavities are not necessary for accurate temperature calibration of surface sensors such as TFRTDs, since such sensors need only be in intimate contact with an object to provide accurate temperature measurement. However, in the case of TFRTDs, cavities are commonly employed for purposes of mounting stability.
The substrates typically employed for the sensors, including glass, ceramic and semiconductor wafers, are relatively brittle. Furthermore, semiconductor wafers are generally single crystals that are subject to breakage from mechanical intrusion. The provision of cavities in a substrate therefore makes the substrates much more fragile then would otherwise be the case, and reduces their manufacturing yield.
Another problem is that the mechanical bond of the sensor to the substrate is often easy to break. This is because the bonding materials presently employed can only bond by entrapment or polymeric adhesion, and also because each lead wire is typically connected to the substrate only at one or two positions, making it easy to dislodge the sensors from their cavities by stresses applied to their lead wires. As a result, systems that employ more than five sensors on a substrate often can be used only once before one or more of the sensors become loosened or dislodged.
The lead wires need to be sheathed to prevent them from shorting out by contact with one another, and also to prevent them from chemically reacting with most substrates. However, currently available sheathing adds significant weight to the lead wires, and can contribute particulate contamination to high temperature process environments.
When the lead wires extend across the surface of the substrate, where the temperature is typically highest, they are subject to reaction with the surrounding environment. Thus, the environment within which a particular structure can function without damage is limited by the resistance of the lead wire material to reaction with and penetration by the surrounding environment. For example, type K TC wire is stable above 500° C. only in inert and hydrogen environments, while type S TC wire is stable above 700° C. only in inert and oxidizing atmospheres. Furthermore, none of these lead wires are stable in reactive process gas environments.
The Al2O3/SiO2 mixtures used for bonding the sensors to the substrate tend to yield only glazes, and generally do not react aggressively with most oxidizable surfaces such as silicon of gallium arsenide wafers. The bond securing the sensor to the substrate tends to be relatively weak.